Mechanical ventilation is an important tool in the management of respiratory failure. However, the ventilation strategy can initiate or exacerbate lung injury. Several mechanisms have been implicated in ventilator-induced lung injury (VILI), particularly cyclical airway collapse and reopening. Inferential data supports cyclical airway collapse as an important factor in the genesis of VILI, but it has never been definitively documented. Strategies minimizing alveolar over distension and cyclical airway closure decrease mortality in ARDS patients but are confounded by hypercapnic acidosis, itself a potential modulator of lung injury. Establishing the presence of cyclical airway collapse, its abolition with palliative interventions, and correlation with severity and distribution of injury is vital to understanding VILI and developing relevant strategies to prevent it. This proposal will evaluate mechanisms by which mechanical ventilation induces lung injury and initiates inflammation in a rabbit model of VILI caused solely by mechanical forces. I will address the following questions: 1) Is cyclical airway collapse and reopening a stimulus for inflammation? 2) Where in the lung and on which cell types do mechanical forces exert their effects? 3) Can we definitively document cyclical airway closure? 4) If so, can we accurately predict ventilatory conditions that prevent it? 5) Do non- mechanical forces (hypercapnia) play a protective role in VILI? Methods: Spatial inflammation distribution measured with monoclonal antibody ELISA and immunohistochemistry. Spatial injury measured by extravascular albumin accumulation and gravimetrics. Regional V and Q measured with aerosolized and perfused fluorescent microspheres. Global gas exchange measured with the multiple inert gas elimination technique. Ventilation heterogeneity measured with multiple breath nitrogen washout. Aim 1A: Examine how posture change and PEEP effect severity and distribution of injury and inflammation. Aim 1B: Determine the anatomical location and cell type(s) responsible inflammation in VILI. Aim 2A : Measure effects of hypercapnia on lung injury and inflammation during VILI. Aim 2B: Examine how hypercapnia alters hypoxic pulmonary vasoconstriction and reduces VILI. Aim 2C : Examine how hypercapnia effects ventilation heterogeneity and reduces VILI. Aim 3A: Document cyclical airway collapse and reopening via changes in regional ventilation distribution. Aim 3B: Validate a method to estimate appropriate PEEP level to prevent cyclical airway collapse.